Advances in electronics have given us pocket calculators, digital watches, heart pacemakers, computers for industry, commerce and scientific research, automatically controlled production processes and a host of other applications.
These have become possible largely because we have learned how to build complete circuits, containing millions of electronic devices, on a tiny wafer of silicon no larger than 25-40 mm square and 0.4-0.5 mm thick. Microelectronics is concerned with these miniaturized integrated circuits (ICs), or "chips" as they are called. In a circuit, electrical energy is supplied from, for example, a microbattery or a double-layer capacitor (DLC) and is changed into other forms of energy by appliances in the circuit, which have resistance.
Recently, with the tendency of miniaturizing of small-sized electronic devices, there have been developed thin-film microbatteries, which have several advantages over conventional batteries, since battery cell components can be prepared as thin (1-20 .mu.m) sheets built up as layers. Usually, such thin layers of the cathode, electrolyte and anode are deposited using direct-current and radiofrequency magnetron sputtering or thermal evaporation.
The area and thickness of the sheets determine battery capacity and there is a need to increase the total electrode area in a given volume. Thin films result in higher current densities and cell efficiencies because the transport of ions is easier and faster through thin-film layers than through thick layers.
U.S. Pat. Nos. 5,338,625 and 5,567,210 describe thin-film lithium cells, especially thin-film microbatteries having application as backup or primary integrated power sources for electronic devices and method for making such. The batteries described in these references are assembled from solid state materials, and can be fabricated directly onto a semiconductor chip, the chip package or the chip carrier. These batteries have low energy and power. They have an open circuit voltage at full charge of 3.7-4.5 V and can deliver currents of up to 100 .mu.A/cm.sup.2. The capacity of a 1 square cm microbattery is about 130 .mu.A/hr. These low values make these batteries useful only for very low power requirements in some microelectronic circuits.
A double-layer capacitor (DLC), as opposed to a classic capacitor, is made of an ion conductive layer between two electrodes. In order to make an electric double-layer capacitor smaller and lighter without any change in its capacitance, it is necessary to increase the energy. This may be accomplished, for example, as described in U.S. Pat. No. 5,754,393, by increasing the working voltage by use of an electrolyte having a high decomposition voltage.
Advanced etching technologies, such as reactive-ion etching (RIE), electron-cyclotron-resonance (ECR) etching and inductively coupled plasma (ICP) etching have been developed to etch semiconductor devices having extremely small features sizes. By using the ICP technique it is possible to etch small diameter through-cavities such as through-holes with a very high aspect ratio and smooth surfaces in a substrate such as a silicon wafer.
The present invention is based on a novel approach, according to which a thin-film micro-electrochemical energy storage cell (MEESC) such as a DLC or a microbattery is created on a macroporous substrate, thus presenting increased capacity and performance. By using the substrate volume, an increase in the total electrode area per volume is accomplished. The cavities within a substrate are formed by deep wet or dry etching of the substrate. For example, holes may be formed by an Inductive Coupling Plasma (ICP) etching using the Bosch process described in U.S. Pat. No. 5,501,893.